Oxynitride glasses and glass-ceramics, i.e., glass and glass-ceramic products wherein nitrogen has been substituted for a portion of the oxygen in the base composition, are known to the art. Nitridation of glasses and glass-ceramics has been observed as increasing the density, viscosity, hardness, and dielectric constant of the base materials.
U.S. Pat. No. 4,097,295 (Chyung et al.) provides a broad disclosure of SiO.sub.2 -based, oxynitride glasses which, being thermally crystallizable, constitute precursor materials for the production of glass-ceramic articles. Hence, SiO.sub.2 and nitrogen, the latter being conjectured as present as the nitride ion N.sup.-3, comprise at least 50% by weight of the compositions and are present in amounts of about 40-85% SiO.sub.2 and 2.5-17% N. The remainder of the compositions consists of network formers and/or modifiers (defined as M.sub.x O.sub.y) which permit glass articles to be formed utilizing conventional glassmaking techniques. The patentees explicitly noted the operability of the alkali metal oxides, the oxides of the Group IIA and Group IIB elements of the Periodic Table, B.sub.2 O.sub.3, and Al.sub.2 O.sub.3. At least 15% of M.sub.x O.sub.y ingredients will be present wherein M.sub.x O.sub.y consists of 10-40% Al.sub.2 O.sub.3 and 5-25% of at least one oxide selected from the group consisting of the alkali metal oxides, the oxides of the Group IIA and Group IIB elements of the Periodic Table, and B.sub.2 O.sub.3.
Silicate glasses containing a rare earth metal oxide from the lanthanide series of rare earth metals are well recognized in the glass art for exhibiting high hardness and elastic modulus values. Although U.S. Pat. No. 4,097,295 makes no reference to such glasses, more recent researchers have investigated the potential for nitriding such glasses to obtain materials displaying extremely high hardness and strength values. Thus, Makishima, Mitomo, Tanaka, and Tsutsumi have published the following two articles disclosing their preparation of a lanthanum silicate glass containing about 18 atomic percent nitrogen, which corresponds to an overall approximate glass composition of 2La.sub.2 O.sub.3 .multidot.SiO.sub.2 .multidot.Si.sub.3 N.sub.4 : "Preparation of La-Si-O-N Oxynitride Glass of High Nitrogen Content," Yogyo Kyokaishi, 88 [11] 701-702 (1980), and "Microhardness and Transparency of an La-Si-O-N Oxynitride Glass," Communications of the American Ceramic Society, C-55-56 March 1983. That glass did, indeed, demonstrate extreme hardness values, viz., a Vickers hardness of 12.0 GPa. Nevertheless, the glass required very high melting temperatures, i.e., in the neighborhood of 1700.degree. C., and was melted in batches of only two grams. Such melting temperatures, coupled with the inherent instability of the glass, render difficult the scaling up required to provide commercially viable volumes of glass utilizing techniques and equipment relatively familiar to the glassmaking art.
Therefore, the primary objective of the present invention was to discover nitrided glass compositions exhibiting good stability containing at least one rare earth metal oxide which demonstrate high hardness values and which can be melted and formed at temperature and with equipment capable of producing large volumes of glass bodies with relative ease.